This invention relates to an exposure-controlling device for an electrophotographic copying machine and more particularly to a device in an electrophotographic copying machine for eliminating unwanted black areas at the edges of an image.
A conventional electrophotographic copying machine contains a photosensitive body which is preliminarily charged by a charger and on which an electrostatic latent image is formed by an exposure to light. A positive image is formed by means of a toner which is oppositely charged and is transferred to a copy paper. The transferred image is fixed by heating and this is how the copying process is effected. With a conventional copying machine of this type, the light-exposure process is effected by moving either the original document or an optical unit with respect to the other such that a lamp which forms a part of the optical system scans the document. During this process, the document is usually covered by what may be referred to as a document cover but if the document to be copied is very thick or bulky such as a bound book, the document cover may float above the table on which the document is placed. This has the effect of reducing the quantity of light reflected by the document cover and hence of forming an undesirable electrostatic latent image on the photosensitive body, this in turn causing black areas to appear on the copy paper. This common phenomenon is illustrated in FIG. 7 which shows unwanted black areas 9c at the front and back edges of a copy in the direction of scan. These black areas 9c come about because, when a bulky document such as a bound book is placed on the document table, the distance along which the book contacts the document table becomes smaller than the actual size of the book. Black areas 9b also appear as shown where the amount of reflected light is reduced. Such unwanted black areas also appear if the original document is moved when the document cover is closed after the document is correctly positioned on the document table.